1. Field of the Invention
This invention relates to devices for detecting power line disturbances and more particularly to devices for analyzing high frequency impulses that occur on AC (alternating current) power lines.
2. Description of Prior Art
When a temporary and intermittent event occurs on an alternating current (AC) power system with a frequency content that is orders of magnitude above the fundamental frequency of the system, it is commonly called an impulse. Impulses may be caused by lightning, inductive loads turning off, capacitive loads turning on, static electricity, and wiring faults, as well as other less common faults. Impulses can both disrupt and damage sensitive loads.
There are many commercially available instruments for measuring parameters of power line impulses. These parameters may include time of occurrence, peak voltage amplitude, rise time, and oscillographic signature. One such commercially available instrument is the Basic Measuring Instruments Model 8800 PowerScope.
An impulse parameter that is especially useful for diagnosing the cause of an impulse is its direction: that is, the direction one must travel from the measuring point along the measured conductor in order to find the source of the impulse. For example, if the impulse is measured at the AC power service entrance of a building, this parameter can indicate whether the impulse originated inside the building or on the utility distribution grid. Portable impulse direction measurements also allow an investigator to track down the source of a series of impulses by judicious selection of measurement sites.
Techniques for impulse direction measurements have been the subject of active investigation for almost twenty years. One technique for determining impulse direction is to measure the polarity of a voltage impulse, while simultaneously measuring the polarity of its associated current impulse. If the polarities match, the impulse was travelling in one direction; if the polarities are opposite, the impulse was travelling in the other direction. This technique is the basis of Gunn et al. in U.S. Pat. No. 3,753,089; Huang in U.S. Pat. No. 3,986,115; and Smith et al. in U.S. Pat. No. 3,986,116. Cox in U.S. Pat. No. 4,187,461 measures the polarities of the leading edges of a voltage impulse and its associated current impulse, then proceeds to compare the polarities in the same way.
However, practical, accurate, and reliable implementations of these techniques have been elusive. One barrier has been the difficulty of making accurate impulse current measurements on AC power lines. On 60 Hertz power lines, impulses typically have frequency content in the 100 kiloHertz to 2 megaHertz range. Standard current transformers with cores and windings optimized for 60 Hertz measurements perform poorly at 2 megaHertz. On the other hand, cores and windings that are optimized for 2 megaHertz may saturate in the presence of large 60 Hertz magnetic fields. Consequently, accurately and unambiguously measuring impulse direction by comparing the polarity of corresponding voltage and current impulses has been difficult outside of the laboratory.
It is the object of the present invention to measure impulse direction without requiring high frequency current measurements.